The 'micro-tasimeter,' a delicate thermoscope, was constructed in 1878, and is the outcome of Edison's experiments with the carbon button. Knowing the latter to be extremely sensitive to minute changes of pressure, for example, those of sonorous vibrations, he conceived the idea of measuring radiant heat by causing it to elongate a thin bar or strip of metal or vulcanite, bearing at one end on the button. To indicate the effect, he included a galvanometer in the circuit of the battery and the button. The apparatus consisted of a telephone button placed between two discs of platinum and connected in circuit with the battery and a sensitive galvanometer. The strip was supported so that one end bore upon the button with a pressure which could be regulated by an adjustable screw at the other. The strip expanded or contracted when exposed to heat or cold, and thrust itself upon the button more or less, thereby varying the electric current and deflecting the needle of the galvanometer to one side or the other. The instrument was said to indicate a change of temperature equivalent to one-millionth of a degree Fahrenheit. It was tested by Edison on the sun's corona during the eclipse observations of July 29, 1875, at Rawlings, in the territory of Wyoming. The trial was not satisfactory, however, for the apparatus was mounted on a hen-house, which trembled to the gale, and before he could get it properly adjusted the eclipse was over. It is reported that on another trial the light from the star Arcturus, when focussed on the vulcanite, was capable of deflecting the needle of the galvanometer. When gelatine is substituted for vulcanite, the humidity of the atmosphere can also be measured in the same way.